Please enter valid non-zero values.
RESULTS
Input Parameters Specification
Reference VoltageEnter regulator reference voltage such as 1.25 V for LM317-style adjustable regulator circuits.
Output VoltageEnter the target regulated output voltage required from the adjustable power supply.
R1 Unit SelectorChoose Ω, kΩ, MΩ or GΩ for the input R1 value only.
R2 Unit SelectorChoose separately whether the R2 result should display in Ω, kΩ, MΩ or GΩ.
Practical Operational Examples
5 V Regulator Example
Reference: 1.25 V
Output: 5 V
R1: 1 kΩ
R2: 3 kΩ
Separate Unit Example
R1 can be entered in kΩ while R2 result can be displayed in Ω, kΩ, MΩ or GΩ.
LM317 Output Setup
For a common adjustable regulator, use the reference voltage from the datasheet and select practical resistor values.
Power Supply Repair
Use the divider ratio to understand how a feedback resistor change moves the regulator output voltage.
Diagrams & Theory
An adjustable voltage regulator uses a feedback resistor divider. R1 connects from output to adjust node, while R2 connects from adjust node to ground.
Formulas & Mathematical Logic
Original formula preserved: R2 = (Output Voltage - Reference Voltage) × R1 / Reference Voltage
In symbols: R2 = (Vo - Vr) × R1 / Vr
R1 input unit is converted internally to Ω. R2 output is converted only for display using the R2 result selector.
Divider Ratio = Output Voltage / Reference Voltage
This adjustable voltage regulator resistor divider calculator keeps the original formula behavior, including negative R2 when output voltage is lower than reference voltage.
Step-by-Step Example
Example values: Reference Voltage = 1.25 V, Output Voltage = 5 V, R1 = 1 kΩ.
Step 1: Convert R1 to ohms: 1 kΩ = 1000 Ω.
Step 2: R2 = (5 - 1.25) × 1000 / 1.25.
Step 3: R2 = 3.75 × 1000 / 1.25 = 3000 Ω.
Step 4: If R2 unit is kΩ, result displays as 3 kΩ.
Step 5: Divider ratio = 5 / 1.25 = 4:1.
How to Use This Calculator
Enter the regulator reference voltage. For LM317-style circuits, this is commonly 1.25 V.
Enter the target output voltage.
Enter R1 and choose its unit from Ω, kΩ, MΩ or GΩ.
Click Calculate to find R2 and divider ratio.
Use the R2 selector to display the result in the unit you want.
About This Calculator
Design the feedback divider for adjustable voltage regulators quickly.
The CalcBoy Adjustable Voltage Regulator Resistor Divider Calculator finds R2 from reference voltage, target output voltage and R1.
Adjustable voltage regulators use a feedback divider to set the output voltage. In many regulator circuits, R1 is placed between output and adjust node, while R2 is placed between adjust node and ground. The regulator keeps the adjust/reference node at a fixed reference voltage, so the resistor ratio defines the final output voltage.
This calculator is useful for LM317-style linear regulators, adjustable power supply modules, bench supply feedback networks, SMPS feedback experiments and general voltage regulator repair. It lets you enter R1 in Ω, kΩ, MΩ or GΩ and separately display R2 in the most convenient unit. That makes it easier to compare calculated values with real resistor series values.
For practical design, choose standard resistor values close to the calculated R2 and then verify the final output voltage. Real circuits may also include adjust pin current, tolerance, load current, dropout voltage, regulator stability capacitors, PCB leakage and resistor accuracy. For precision supplies, use low-tolerance resistors and confirm output with a meter.
Best UseAdjustable regulator feedback resistor design.
Supported InputsReference voltage, output voltage and R1 with unit selector.
Helpful ForLM317 circuits, power supplies, SMPS feedback and regulator repair.
Design ReminderCheck real output after selecting standard resistor values.
Tip: For LM317-type regulators, 240 Ω is often used as R1 in datasheet examples, but your circuit may use a different value. Always follow the regulator datasheet.
Frequently Asked Questions
What does this calculator find?
It calculates the R2 resistor value for an adjustable voltage regulator feedback divider.
What is reference voltage?
Reference voltage is the regulator’s internal feedback reference. For many LM317-style circuits, it is around 1.25 V.
Can I use different units for R1 and R2?
Yes. R1 has its own unit selector, and the R2 result can be displayed separately in Ω, kΩ, MΩ or GΩ.
Does this include adjust pin current?
No. This calculator preserves the simple divider formula from the source code and does not add adjust pin current correction.
Why is R2 negative sometimes?
If output voltage is lower than reference voltage, the preserved formula returns a negative R2 value.
Should I use exact calculated resistor values?
Use the nearest standard resistor value, then verify the real output voltage in the circuit.
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